1. Field of the Invention
The present invention relates in general to sense amplifiers for semiconductor read-only memories (ROMs). In particular, the present invention relates to a multi-stage sense amplifier for ROM devices capable of storing more than one bit of data in one single memory cell unit.
2. Technical Background
ROM's are widely used semiconductor integrated circuit (IC) memory storage devices for digital electronic equipment, and, in particular, microprocessor-based computer systems. Conventional digital circuitry in ROM's employ sense amplifiers to sense the content of an addressed memory cell in order to "read" a memory location.
State-of-the-art semiconductor ROM IC devices, however, store a single binary bit of information in each of their memory cell units. That is to say, the sensed result with respect to each memory cell unit of the ROM device is identified between two electrical potential stages. In other words, the memory content of a ROM memory cell as read is either a logical 1 or a logical 0.
Enlargement of the memory storage capacity (by increasing the number of memory cells), as well as reducing the physical size of the semiconductor device, have been targets which the semiconductor memory industry has been pursuing. Enlarged memory capacity and reduced memory semiconductor die size represent increased functionality and reduced cost, both of which are positive factors in the merchandising of semiconductor memory products. The greatest effort in the art has been concerned with the reduction of the device dimensions in order to squeeze more memory cell units in the same semiconductor die area. This approach, however, is limited by the current state of the art in resolving details during semiconductor processing.